The invention relates to a circuit arrangement for feeding a discharge lamp, comprising
lamp clamps for holding the discharge lamp, PA1 a main inverter coupled to the lamp clamps for generating, during stationary operation, a current which is fed to the discharge lamp, PA1 an auxiliary inverter for preheating electrodes of the discharge lamp, provided with PA1 a control circuit coupled to the main inverter and the auxiliary inverter for controlling the operating state of the circuit arrangement, PA1 a first circuit part coupled to an input of the control circuit for generating a first signal which is a measure of the voltage difference between a first end of the first secondary winding and a first end of the second secondary winding.
an oscillator for generating an alternating voltage with a frequency f1, PA2 a transformer provided with a primary winding coupled to the oscillator, and with a first and a second secondary winding which each shunt a lamp electrode during operation of the lamp,
Such a circuit arrangement is well-known. After putting the known circuit arrangement into operation, the control circuit ensures that, if a discharge lamp is connected to the lamp clamps, the circuit arrangement is successively brought into a number of operating states. In the first operating state, the lamp electrodes are preheated by means of the auxiliary inverter. Subsequently, in a second operating state, an ignition voltage is generated across the discharge lamp by means of the main inverter. If the discharge lamp ignites under the influence of this ignition voltage, the control circuit brings the circuit arrangement into a third operating state wherein the discharge lamp is fed so as to remain in the stationary mode of operation. The first signal, which is a measure of the voltage difference between a first end of the first secondary winding and a first end of the second secondary winding, represents the voltage across a discharge lamp connected to the circuit arrangement. The first signal is used by the control circuit to preclude that the voltage across the discharge lamp becomes too high during ignition, and to establish whether the discharge lamp has ignited.
As mentioned hereinabove, it is first checked whether a discharge lamp is present. For this purpose, the known circuit arrangement also comprises means for establishing whether a discharge lamp is connected to the lamp clamps. These means generally include a circuit part which generates a current which flows through one of the lamp electrodes and is subsequently detected. The detection, or non-detection, of this current affects the form of a lamp-presence signal which is present at an input of the control circuit. If said lamp-presence signal indicates that no discharge lamp is connected to the circuit arrangement, the control circuit keeps the circuit arrangement in a state of rest. A drawback of the known circuit arrangement resides in that the control circuit must be provided with an input where the lamp-presence signal is present and which input is used exclusively to determine whether a discharge lamp is connected to the circuit arrangement. Since the control circuit often comprises an IC, the total number of inputs and outputs of the control circuit is determined to a substantial degree by the number of pins of the IC. In the known circuit arrangement, the number of pins of the IC is relatively large in the control circuit. As a result, the control circuit is relatively expensive and difficult to manufacture.